1. Field of the Invention
The present invention relates to inductor circuits, and in particular, to such circuits that allow variable inductances.
2. Related Art
As is well known, inductors generate an electromotive force by changing the current flowing through the inductor. In its simplest form, an inductor is a coiled conducting wire with N number of turns. Inductors can be used in a wide range of applications, such as for impedance matching and loading in low noise amplifiers, broadband amplifiers, power amplifiers, mixers and analog filters, as well as for providing frequency-selective resonant circuits in variable-frequency oscillators in such RF-wireless products. However, with many applications, as the frequency of operation of a device increases, the circuit becomes more complicated, and narrow variation and strict tolerance are required for the components to operate properly. Thus, it is desirable to be able to adjust the parameters of the circuit, such as the inductance, to finely tune for variations in the components as well as to allow variable operation of the circuit. Furthermore, in certain applications, it is desirable to vary the amount of inductance in a circuit. For example, in a circuit that has a time varying capacitive load or a varying frequency of operation, the circuit may be tuned by varying the inductance to minimize the reactive current required to be supplied by a power source.
Conventionally, active inductors are used in integrated circuits that require tuning or varying the inductance values. However, active inductors have several disadvantages, including degraded noise performance, higher distortion, relatively high power requirements, lower quality or Q factor (i.e., the relative absence of resistive losses) than passive inductors, and limited dynamic range. In addition, active inductors are limited in their operation to lower speeds and are not capable of providing variable inductance at high speed.
Passive inductors provide some advantages over active inductors, such as higher quality factor, better noise performance, and no extra power consumption. Passive inductors also provide better performance with high speed circuits and have wideband inductance up to their self-resonant frequency. However, changing the inductance of passive inductors is often burdensome. Inductance is typically changed by altering its physical characteristics, such as length, diameter, number of turns, and wire thickness. However, once those physical characteristic are set, the inductance if fixed, such that any change would require changing a physical characteristic, such as altering the separation between coils, or switching in a new inductor. Inductance can also be varied by connecting wires, commonly known as taps, at various locations throughout the coil and extending the wires to contact switches located at some distance from the coil. However, the use of many taps may also result in the formation of undesirable parallel resonance circuits and a lowered overall Q factor for the circuit, thereby reducing the sharpness of the circuit response at the selected frequency.
Accordingly, there is a need for a variable inductor that overcomes the deficiencies in the prior art as discussed above.